Embryonic and fetal development in a commercial dam-line genotype

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304 S.C. Town et al. / Animal Reproduction Science 85 (2005) 301–316production. These gilts were then mated with crossbred PIC terminal-line 337 boars atproduction level. Bred females from one such commercial breeding unit (ADL 1 Swine Unit,Osceola, IA, USA) were made available for this study as part of a herd depopulation exerciselinked to mortality in nursery pigs from the sow unit due to previous PRRS outbreaks.Productivity of these sows had previously been adversely affected by infection with twodifferent strains of PRRS virus. However, PRRS virulence within the breeding herd wasstabilised by vaccinations with autogenous PRRS vaccines in July and early December2000, at least 60 days before any of the sows used in the present study were bred.As part of the study design, weaned sows were bred by AI with semen from Landrace orLarge White Boars (Pig Improvement Company, USA) according to normal herd practice. Inaddition, a group of gilts was also specifically bred to be included in the parity 0–1 data set.Females were inseminated each morning that standing oestrus was detected in the presenceof a boar, using pooled semen containing approximately 4 billion sperm per AI dose. Bredfemales were checked daily for return to oestrus in the presence of a boar until day 50 ofgestation, unless they were allocated to an earlier slaughter group, in which case they werechecked for oestrus until the day they were shipped. For the purposes of this study, 454presumed pregnant females were initially allocated to slaughter on one of three days on thebasis of parity; gilts and first parity (parity 0–1), second and third parity (parity 2–3), andfourth parity and greater (parity >4). The parity distribution of animals shipped is shownin Fig. 1. Of the approximately equal numbers of sows initially allocated to be at differentstages of gestation at the time of slaughter, data were obtained from the reproductive tractsof 103 of 114 animals at day 20–30, 149 of 163 animals at day 50–55 and 169 of 177animals at day 85–90 of gestation (total of 421 pregnant tracts at slaughter). Mishandlingand loss of tissue during dissection resulted in some loss of material; the numbers of tractscontributing data to different reproductive characteristics within different parity groups areshown in Table 1. Logistical constraints to measuring reproductive tract data from sucha large number of animals over a period of only 3 days prevented the identification ofindividual sows. Given this constraint, the rationale behind the parity group allocations wasthat in gilts and first parity sows (parity 0–1) ovulation rate and embryonic survival rateFig. 1. Parity distribution of sows allocated for slaughter (N = 454).
S.C. Town et al. / Animal Reproduction Science 85 (2005) 301–316 305Table 1Reproductive characteristics of sows (least square means ± SEM)ParameterParity groupOvulation rate (N = 405)0–1 2–3 >420.18 ± 0.49 a(n = 138)23.59 ± 0.43 b(n = 121)24.71 ± 0.38 b(n = 146)Number of viable conceptuses N/A 13.65 ± 0.39 a 11.81 ± 0.34 bDay 20–30 sows (N = 80) n = 0 n = 37 n = 43Placental weight (g) at day 20–30 N/A 7.55 ± 0.43 a 3.42 ± 0.40 bFetal weight (g) at day 20–30 N/A 0.46 ± 0.03 a 0.40 ± 0.03 aPlacental efficiency at day 20–30 N/A 0.07 ± 0.01 a 0.12 ± 0.01 bDay 50–55 sows (N = 149) n = 71 n = 31 n = 47Day 85–90 sows (N = 166) n = 57 n = 48 n = 61Placental weight (g) at day 85–90 235.7 ± 7.3 a 209.5 ± 8.5 b 235.4 ± 7.1 aFetal weight (g) at day 85–90 751.8 ± 19.4 a 755.8 ± 22.3 a 738.7 ± 18.7 aPlacental efficiency at day 85–90 3.30 ± 0.09 a 3.68 ± 0.11 b 3.23 ± 0.09 aBrain:liver weight ratio at day 85–90 1.00 ± 0.03 a 0.85 ± 0.04 b 0.92 ± 0.03 a,bGestation dayNumber of viable conceptuses(parities 2–3 and >4)Survival rate (%) parities 2–3 and >4(N = 254)20–30 (n = 80) 50–55 (n = 74) 85–90 (n = 100)14.98 ± 0.46 a 12.17 ± 0.48 b 11.05 ± 0.41 b61.8 ± 2.1 a 50.2 ± 2.2 b 48.7 ± 1.9 bLeast square means ± SEM within a row with different letters differ (P ≤ 0.05). N/A denotes data not availablefor analysis.would typically be limiting factors for litter size born. In contrast, in second and third paritysows (parity 2–3) reproductive performance would be optimal and uterine capacity, ratherthan ovulation rate and embryo survival rate would be the critical constraint on litter size.Sows of fourth parity and greater (parity >4) would represent sows in which reproductiveperformance would be declining, associated with increased variability in the uniformity ofpigs born in terms of numbers and development.2.2. Slaughter and dissection proceduresReproductive tracts were dissected at cooperating commercial abattoirs immediately afterslaughter (PorkKing Packing Company, Marengo, IL; Abbyland, Curtis, WI) and ovulationrate and number of viable conceptuses were recorded for all animals. A requirement ofthe depopulation process was to ship all animals over a 3-day period. Time and manpowerconstraints limited the number of conceptuses that could be dissected from each reproductivetract. Therefore, day 20–30 and 85–90 were chosen as critical time points to monitorany effects on development. Unfortunately, these logistical constraints did not allow collectionof embryo data from day 20–30, parity 0–1 animals, which resulted in a missing
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Embryonic and fetal development in a commercial dam-line genotype

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